The number of users of the existing wireless communication systems, such as e.g. Global System for Mobile communication (GSM), is constantly increasing. This infers an increased use of the resources of the wireless communication systems. In order to meet the continuous increase of use of the resources, the concept of using Voice services over Adaptive Multi-user channels on One Slot (VAMOS) has been introduced into the GSM standard. According to this concept, the network may multiplex data to/from two wireless transmitting units and/or receiving units. In other words, the two transmitting/receiving units are allocated the same radio resource.
The sub-channels are separated by means of orthogonal training sequences. In the uplink the two sub-channels are not orthogonal, and complex joint detection algorithms are required in order to demodulate the users' data bits. Further, only two users may share the same radio resource at any given time.
Since uplink transmissions are uncoordinated, this solution is based upon co-channel signals that interfere strongly with each other, causing degradation of the link performance. Even in the downlink, each mobile station experiences the paired sub-channel as interference that must be suppressed.
Relevant prior art is disclosed in 3GPP TS 45.001 V9.2.0 (2010-03); Radio Access Network; Physical Layer on the radio path; General Description (Release 9), and in 3GPP TS 45.004 V9.1.0 (2010-05); Radio Access Network; Modulation (Release 9).